It is important to note the difference between of and from when passing an array-like structure (including strings):

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Observable.of([1, 2, 3]).subscribe(x => console.log(x));

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would print the whole array at once.

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On the other hand,

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Observable.from([1, 2, 3]).subscribe(x => console.log(x));

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prints the elements 1 by 1.

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For strings the behaviour is the same, but at character level.

They both transform values into observables except 'from' will convert from:

- a subscribable object, 

- a Promise

- an Observable-like

- an Array

- an iterable or an array-like object to be converted

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'of' will take any value!